Dinner for group near convention center in Phoenix
As suggested above, Los Dos Molinos might work. It's fiery New Mexican food, although your user name suggests that's nothing new for you. Some people say LDM is overrated, and they're probably right, but it would still work for your group and is walkable from the convention center.
Even close to the Convention Center is Sam's Cafe, which serves tourist-friendly Southwestern lite. This place is not as good as it was in the '90s, but still okay.
Two options involving some travel:
Herd your students onto the westbound light rail, ride up to Osborn, and then walk a few blocks to Gallo Blanco at the Clarendon Hotel. Of all the restaurants mentioned in my reply, Gallo Blanco's interior Mexican food is generally regarded as the best, and the prices are quite reasonable.
You could also go via cab or the #10 bus over to Tradiciones, a Mexican restaurant adjacent to the Phoenix Ranch Market. They'd be likely to handle your group well. If a more casual experience is desired, your group could also eat in the food court in the adjacent market.
1602 E Roosevelt St, Phoenix, AZ 85006
455 N 3rd St Ste 114, Phoenix, AZ 85004
Los Dos Molinos
1010 E Washington St, Phoenix, AZ 85034
Gallo Blanco Cafe
401 W Clarendon Avenue, Phoenix, AZ 85013